An MRI apparatus is an apparatus which measures an NMR signal generated by an object, especially, nuclear spins which form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding is given to the NMR signals by a gradient magnetic field and frequency encoding is also given to the NMR signals, and the NMR signals are measured as time-series data. The measured NMR signals are reconstructed as an image by two-dimensional or three-dimensional Fourier transform.
Generally, in the MRI apparatus, only a one-dimensional direction of the object is specified and an arbitrary planar region with a predetermined thickness is selectively excited using a high-frequency magnetic field (hereinafter, referred to as an RF) and the gradient magnetic field. In contrast, there is a two-dimensional spatial selective excitation (spectral-spatial; hereinafter, referred to as SS) method of specifying two directions instead of the entire planar region and exciting selectively only the inside of a region limited by the two directions (for example, refer to NPL 1). In the SS method, in order to realize such two-dimensional spatial selective excitation (hereinafter, simply referred to as two-dimensional excitation), waveforms of the RF and the gradient magnetic field are calculated on the basis of a profile of an excitation region, and the calculated RF for the two-dimensional excitation (hereinafter, referred to as a 2DRF) is applied together with an oscillating gradient magnetic field.
Since a signal can be acquired by exciting only the inside of the region selected by restriction in the two-dimensional direction in the SS method, a signal from the outside of the region can be effectively suppressed. For example, this SS method is used for a navigator echo sequence (hereinafter, referred to as navi-echo) for tracking the movement of the diaphragm (for example, refer to NPL 2). In the navi-echo, the vicinity of the diaphragm is excited in a cylindrical shape in a body axis direction using the SS method, and a temporal change of a diaphragm position in a cylinder axis direction of a region excited in the cylindrical shape is detected from echo signals generated from the region to thereby monitor a respiratory motion.